1. Field of the Invention
This invention relates to improvements in apparatus for spraying a powder circumferentially around a pipe joint in a pipe or the like. More particularly, this invention relates to an improvement in the method and means for supplying the air-powder suspension to the weld joint and for returning excess powder to the system.
2. Description of the Prior Art
U.S. Pat. No. 4,038,942 entitled "System for Spraying a Powder Circumferentially Around a Pipe", and owned by the assignee of the present invention, describes a yoke, a powder dispensing device mounted on the yoke, a powder suspension system, a means for delivering powder from the powder suspension system to the powder dispenser and also means for returning excess powder from the powder dispenser back into the system. The powder suspension device shown in U.S. Pat. No. 4,038,942, referred to above, is similar to that described in U.S. Pat. No. 3,854,634 entitled "Powder Dispenser". The yoke employed in U.S. Pat. No. 4,038,942, referred to above, is similar to that disclosed in U.S. Pat. No. 4,005,677 entitled "Apparatus for Circumnavigating a Dispenser About a Pipe or the Like".
The pipe or pipeline to which the prior art and the present invention both relate, is generally made up from pipe sections which have been previously coated at the mill except for the ends thereof which are left uncoated so that the sections can be welded together in the field in end-to-end relation. The apparatus in U.S. Pat. No. 4,005,677, therefore, deposits powder on the weld joints (which have been previously heated) to complete the coating of the pipe. However, it is difficult to meter exactly the quantity of powder to be supplied to each weld joint. Obviously, an insufficient quantity of powder will result in a poor or inadequate coating of the weld joint. In practice, a quantity of powder slightly in excess of that required is supplied to the weld joint. This excess powder can result in the contamination of the air in the area of the pipe joint, and it can also result in a loss of powder from the system. U.S. Pat. No. 4,038,942 provides a system whereby the excess powder in the area of the weld joint is sucked up and returned to the system.